Big Bang Poured Out "Liquid" Universe, Atom Smasher Hints

A picture shows the tracks of particles produced in a lead ion smashup in the Large Hadron Collider.

Image courtesy ALICE/CERN

In the immediate aftermath of the big bang, the universe behaved like a very dense, superhot liquid, according to data from the most powerful atom-smashing experiment yet performed.

Physicists recently re-created the conditions of the big bang using the ALICE detector in the Large Hadron Collider (pictures) near Geneva, Switzerland. The scientists smashed together lead ions—atoms of lead that had been stripped of their electrons—at nearly the speed of light.

The experiment successfully created a tiny "subatomic volume" of a primordial state of matter known as a quark-gluon plasma. This exotic substance is thought to have existed only briefly in the early universe.

The plasma is made of subatomic particles called quarks and gluons. Quarks are the elementary building blocks of positively charged protons and neutral neutrons, which make up the cores of atoms. Gluons are particles that "glue" quarks together using what's called the strong force.

Previous experiments had shown that at extremely high temperatures, the strong force weakens and quarks and gluons can't join. Some theories had therefore predicted that quarks and gluons would have been widely spaced in the extreme heat of the very early universe, so that the quark-gluon plasma would have behaved like a gas.

Based on the new LHC data, though, "that doesn't seem to be true," said David Evans, a physicist at the University of Birmingham in the U.K. and leader of a team working with the LHC's ALICE detector.

The experiment created quark-gluon plasma at more than 18 trillion degrees Fahrenheit (10 trillion degrees Celsius). The substance existed for only a tiny fraction of a second before cooling down and turning into normal matter. But the plasma was there long enough for Evans and colleagues to see how it behaved.

"Even though the strong force is weaker at these energies, it's still very strong, and the particles are interacting quite a bit, so this system behaves much more like a liquid than a gas," Evans said.

However, it's not like any liquid most people would recognize.

"A pinhead-size drop of this liquid would have the same mass as the pyramids of Egypt," Evans said, "and it's also a million times hotter than the center of sun."

Plasma Cooled Into Too Many Particles?

Quark-gluon plasma has been made before at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York. That plasma also behaved like a liquid, but was only half as hot as the one made at the LHC.

Scientists had hoped that by creating quark-gluon plasma at different temperatures, they might finally see the particles in their predicted, gaslike form. Such data would allow physicists to better understand how the strong force works.

Instead, the LHC's quark-gluon plasma "tells us that perhaps we understand the strong force less than we thought we did," Evans said.

Another hard-to-explain result from the LHC experiment is that the cooling quark-gluon plasma coalesced into significantly more particles than most theories had predicted.

For example, "one theory said there was a maximum amount of gluons that you could fit into a certain volume of space before you had gluon saturation," Evans said.

When this saturation point is reached, no more particles should be created. However, the number of particles created in the LHC experiment exceeded expectations by about 20 percent.

Having hard data about the number of particles created during the quark-gluon plasma cooldown is particularly valuable, because it has implications for better understanding how quark-gluon interactions are mediated by the strong force, said theoretical physicist Ulrich Heinz of Ohio State University.

This work could have practical implications in the future. Understanding electromagnetism on a subatomic scale, for example, allowed scientists to exploit a property called superconductivity, in which certain materials lose all electrical resistance if the temperature is low enough.

Superconductors have a variety of uses, such as the superconducting magnets used in MRI machines and even in the LHC itself.

A better understanding of the strong force could lead to the discovery of a similarly useful phenomenon, Heinz said.

"I'm not saying we'll find something that looks exactly like superconductivity," he said, "but we might find something of that quality."